Dental restorations such as bridges, crowns, dentures, partial dentures, inlays, onlays and the like have employed gold alloy for many years. Because of its high cost, many attempts have been made to make and employ non-precious metal alloys in place of the gold. Such non-precious metal alloy compositions are illustrated, for example, by the U.S. Pat. Nos., 1,736,053; 2,089,587; 2,156,757; 2,134,423; 2,162,252; 2,631,095; 3,121,629; 3,464,817 and 3,544,315. Gold alloys, however, has many advantageous properties as a dental alloy and many of the previously prepared non-precious metal alloys have been found to be unsatisfactory in various respects when compared to the conventional gold alloy.
One of the problems encountered in attempts to use non-precious metal alloys for dental work in place of gold is that many of these alloys have been hard to cast because of a too high melting range. In order to be accepted generally by dental laboratory technicians, the melting temperature of the alloy should not be much in excess of 2400.degree. F. It is desirably within the range of about 2000.degree. to 2350.degree. F., more preferably, nearer the lower limits. A practical reason for this is that many dental laboratories use torches of the gas oxygen type which will not heat to much above 2500.degree. F. so that if higher melting alloys are used then special heating equipment such as oxygen-acetylene torches must be obtained for working the metal. Although attempts have been made to adapt these alloys by modifying casting techniques such as by changing shape, dimensions, number and point of attachment of sprues, by using special investment materials, or by using special after-casting treatments, the advantages of the heretofore available nonprecious metal alloys have not been sufficient to serve toward general acceptance of these alloys as a preferred substitute for gold alloy in dental constructions.
Another problem with many of the heretofore known non-precious metal dental alloys is one of corrosion. The non-precious metal alloys generally are not as resistant as gold with respect to corrosion by mouth acids. Not only does corrosion cause loss of structural metal but in the case of those dental restorations such as porcelain jackets, crowns, bridges, etc., which are faced with porcelain, corrosion may cause, in the case of certain non-precious metal alloys formation of colored ions which discolor the porcelain. Thus, for example, the presence of cobalt, copper or iron in the metal alloy in an appreciable amount tends to discolor the porcelain bonded thereto.
A still further serious problem encountered in constructing a metal core or framework is the difficulty of soldering the non-precious metal alloy parts to themselves or to gold by using the conventional readily available dental solders. Moreover, when conventional solders such as gold alloy solders are used with non-precious metal alloy there is a tendency for galvanic corrosion to occur at the interface. Also, since non-precious metal dental alloy materials heretofore designed for use as structural metals are frequently substantially harder than gold, there is the added disadvantage of greater time and effort which must be spent on grinding the metal core for precise fit after casting.
In addition to the problems relative to the properties of the prior non-precious metal alloy as a general dental alloy, certain additional requirements exist in connection with the use of the dental alloy as a material which is to be faced with tooth enamel simulating material such as porcelain. Thus, the coefficient of expansion must be compatible with that of porcelain. Where there is not the desired compatibility in the coefficient of expansion between the metal and porcelain, fractures may develop in the porcelain during the firing and subsequent cooling. The preferred relationship of porcelain to metal is such that at room temperature there is compression in the porcelain or glass layer and tension in the metal. Further, the fusion temperature of the alloy, while it should not be so high as to be difficult to cast, it must be sufficiently above the firing temperature of the porcelain so that there is no deformation of the metallic core during firing. Moreover, metal alloys must bond adequately to procelain so that when subjected to mechanical stress, there does not occur a separation at the interface in whole or in part.
Thus, it is the object of the present invention to provide for a dental construction such as bridges, crowns, etc., having a metal core of a non-precious metal alloy and a tooth enamel simulating outer covering bonded thereto wherein said non-precious metal alloy is free from the objections enumerated above and moreover the relationship of the physical properties between the metal core and outer covering are such that the foregoing problems are met. The preparation of a non-precious metal dental alloy which is particularly suitable in dental constructions but which may be employed in other dental applications is another object of the present invention. Another object is to provide for a dental construction which employs a non-precious metal alloy which is not only less costly than gold but has advantages over gold as a dental structural material. A further object is the provision of a dental alloy which may be employed without appreciably changing present techniques or equipment.